1. Field of the Invention
The present invention relates generally to aircraft automatic flight control systems and more specifically to the capturing and control of a precomputed descent path.
2. Description of the Prior Art
Most commercial transport aircraft, general aviation aircraft, and military aircraft are equipped with an automatic flight control system. Automatic flight control systems generally provide the human pilot with the capability of altering the flight path of the aircraft.
Many of these aircraft are equipped with either Performance or Flight Management systems. These systems are generally interfaced with the automatic flight control system in such a way as to be able to control the flight path of the aircraft. This capability therefore allows the control of the aircraft to either a desired speed or altitude by adjusting its flight path to an appropriate value.
The aforementioned Performance and Flight Management systems often determine a computed descent path and speed to a predetermined geographical location. The calculation of these descent paths necessarily requires assumptions about the winds that will be encountered during the descent since the exact nature is unknown. Thus, winds that differ significantly from those assumed will cause the aircraft to deviate from the optimum path and thereby either under or overfly the desired geographical location.
Thus, it is desirable to devise some method of control that will capture and maintain the computed descent path. The method used in the prior art is to determine the difference between the aircraft altitude and the altitude of the descent path at the same point, thereby determining an altitude or path error. This error is then used to adjust the flight path angle of the aircraft until the error is null in the well-known method of feedback control systems.
In essence, this scheme is an altitude hold control law wherein the commanded altitude is continually decreasing. As the speed of the aircraft is not controlled directly, either excessively fast or slow speeds may occur during the corrections back to the computed path. To assure the aircraft does not exceed its performance capabilities, it is necessary to continuously monitor the speed of the aircraft and abandon the path correction feature and control the speed of the aircraft within acceptable limits should the speed of the aircraft approach its performance limits. Thus, such schemes are necessarily complex and entail the incorporation of altitude error and speed error control laws and a suitable means of switching between the two. Furthermore, it is generally necessary to have a different control law for capturing the path than is used for tracking the path since the capturing of a path is usually a much more dynamic maneuver than tracking.
The present invention overcomes the shortcomings of the prior art by utilizing an incremental speed command to cause convergence to and tracking of the computed descent path. As noted, Performance and Flight Management systems generally compute both a descent path and a nominal speed for descent. The present invention increases or decreases the commanded speed as a function of descent path error. Furthermore, the incremental speed to be added to the nominal descent speed is rate limited in such a way as to minimize any objectionable pitch maneuvers.